Water Vapor and Pressure

Each type of molecule exerts a different force depending on its mass. Because of this, each different type of molecule contributes its own pressure.
A portion of this pressure is from water vapor so we call this vapor pressure. Because water vapor is the lightest among most atmospheric gases, when found in relatively large quantities, it creates lower pressure.
This allows the air to continue to rise, allowing water vapor to condense into fog, clouds, and precipitation.